Method of picking chromium-iron alloys



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Cross Reference Enem 1 prevents over-etching of the hydrofluoric Patented J an. 24, 1928.

' UNITED "STATES 1,657,228 PATENT 1 OFFICE.

LOUIS scnunrn, or PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO ALLEGHENY srnnr.

COMPANY, or nRacxENRInen, PENNSYLVANIA, A conrona'rroN or rn NsYL- .VANIA.

No Drawing.

This invention relates to chromium iron alloys and more particularly to a method of pickling or removing the scale from such alloys.

The chromium iron alloys to which this mvention is particularly directed are those known as rustless or stainless iron and stainless steel. In such alloys, the chromium content may vary between 8% and and the carbon content may vary 'depending upon the particular use to which articles made therefrom are to be put. The alloy may contain silicon and other metalloids.

.In one alloy with which I have had experience, the chromium content is approximately 12% and the carbon content is approximately .12%. The scale produced on such alloy when the same is either box annealed or open annealed is extremely diificult to remove. The pickling solutions most commonly used contain sulphuric acid or hydrochloric acid and certain brightening agents and these not only will not remove the scale from these alloys but they have a tendency to dissolve the alloy itself in preference to the scale.

An ob'ect of this invention is to produce a metho of pickling or removing the scale from such iron chromium alloys without detrimentally etching or otherwise attacking the alloy itself.

I have discovered that if hydrofluoric acid is used as a preliminary pickle and then after the alloy has been rinsed is followed by a nitric acid ickle the scale will be removed. I have iscovered, however, that if hydrofluoric acid is used alone as the preliminary ickle, it has a tendency to overetch or it the alloy itself. The hydrofluoric aci apparently activates the scale or puts it.in a state where it can and will be acted upon by the nitric acid ickle. After many trials and experiments, Ihave discovered that the scale can be properly activated without over-etching the alloy itself, if a metal or a metal compound that is positive to both iron and chromium is added to the hydrofluoric acid pickle. Potassium, sodium, calcium, aluminum and zinc are examples of such metals.

As a cheap metal that is positive to both. iron and chromium and that satisfactorily chromium are added to t mz' rnon or PICKLING cnnomrUM-InoN annoys.

Application med. June 2,1926. Serial No. 118,886.

acid, I prefer to use commercial nitre cake. The preliminary pickle which I have found satis a'ctory contams from 1% to 6% hydrofluoric acid and from to 3% nitre cake.

Thesolution is preferably used cold and the alloy is immersed therem for a length oft1me which varies according to the heat treating method to which the alloy or the articles made therefrom have been subjected.

A hot rolled sheet can be pickled in a short time, while a hot rolled open annealed sheet takes somewhat longer, and a hot rolled box annealed sheet takes still longer. A hot rolled box annealed sheet re uires about three hours in a cold solution. fter the preliminary pickling and after the alloy or articles made therefrom have been rinsed, about 15 .to 20 minutes pickling in a warm nitric acid solution of from 2% to concentrated will be sufficient to remove the activated scale.

The preliminary pickling can be shortened if the solution is somewhat warmed, but I do not advise a temperature of over 140 F. If the percentage of nitre cake in the preliminary pickle is reduced, the temperature, of the solution may be somewhat raised.

The nitre cake contains about 28% of sulphuric acid but its high ositive sodium content tends to prevent th1s from dissociating thereby preventing overpickling or overetching by this sulphuric acid content.

When metals that are ositive to iron and he hydrofluoric acid pickle, fluorides of these metals are formed and instead of adding such metals to the pickle, it is possible to add fluorides of the metals above mentioned.

An over addition of chlorides must be avoided in order to prevent a too violent action of the pickle.

If it is desired, a combination of sulphldes and chlorides of metals thatare positive to iron and chromium may be added to the hydrofluoric acid pickle.

The reliminary or scale act1vat ng p1ckle must'a ways be followedb a nitr1c acid pickle. The theory that I ave proceeded upon is that if the scale is act1vated, 1t 15 made soluble in nitric acid and I believe that in the step of activating the scale, it is converted into a chromium-iron-fluoride which is readily soluble in nitric acid.

Having thus described my invention, what I claim is a Nev...-

ositive to iron and chromium andvating the scale thereon in a solution containing hydrofluoric acid and the salt of a metal positive to iron and chromium and then in dissolving the scale infa solution containing nitric acid.

In testimony whereof, I have hereunto subscribed my name this 26thday of May, 1926.

LOUIS SCHULTE. 

